1. Field of the Invention
The present invention relates to a power conversion apparatus. More specifically, the present invention relates to the power conversion apparatus for generating on/off signals using pulse width modulation (PWM).
2. Background Information
Several types of voltage-type inverters used to drive alternating current motors are currently known. In these inverters, three of single-phase half-bridge circuits are connected in parallel with each of the single-phase half-bridge circuits having two electrical switches connected in series, and the on/off signals of the switches are generated by pulse width modulation (PWM). When the switches are switched on and off, a ripple current will be generated according to the frequency of the PWM modulating signal (the carrier) and according to the motor inductance. Thus, a smoothing capacitor inserted into the circuit to smooth out the ripple current.
Japanese Laid-Open Patent Application Publication No. 2000-78850 discloses one example of an inverter device and a method for controlling the inverter device that reduces the ripple current and the capacity of the smoothing capacitor. In the inverter device disclosed in this reference, two inverters shares a common smoothing capacitor. Moreover, countermeasures are adopted such as synchronizing the carriers of each inverter, switching the peaks and troughs (valleys) of the triangular waves of the inverters used for modulation, and synchronizing the on state of one with the off state of the other. As a result, the ripple currents flow alternately into the smoothing capacitor to cancel each other.
Moreover, Japanese Laid-Open Patent Application Publication No. 05-56653 discloses another example of an inverter apparatus in which a converter is placed between an alternating current source and a direct current bus, and an inverter is placed between the direct current bus and the alternating current load. In the inverter apparatus disclosed in this reference, the two modulation signals are synchronized, and the flow of current into a smoothing capacitor attached to the direct current bus is reduced. Thus, the inverter apparatus disclosed in this reference performs synchronization control of two power conversion devices (the converter and the inverter) that are connected to the shared smoothing capacitor.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved power conversion system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.